US 5703286 A Resumen A new technique for interpreting pressure data measured during a formation test. The new technique uses an exact spherical flow model that considers the effects of flow line storage and that can be solved in closed, analytical form. This technique generates a type-curve that matches the entire measured pressure plot and that can accurately predict ultimate formation pressure during formation testing from a pressure plot that has not achieved steady state values near the formation pressure.
Reclamaciones(7) 1. A method of testing an underground formation, said method comprising the steps of:
disposing a formation testing device within a borehole adjacent a portion of said underground formation to be tested, said formation testing device having a probe for collecting fluid from said formation and having a transducer for measuring fluid pressure, said transducer being fluidically coupled to said probe by a flow line; drawing fluid from said underground formation through said probe and into said formation testing device, and permitting fluid pressure within said formation testing device to build toward fluid pressure within said underground formation; delivering an electrical signal from said transducer to a signal processor electrically coupled to said formation testing device, said electrical signal being correlative to fluid pressure of said fluid in said formation testing device; generating an electrical plot in response to receiving said electrical signal, said electrical plot being correlative to fluid pressure of said fluid in said formation testing device over time; and generating an electrical type-curve that approximates said electrical plot wherein said step of generating an electrical type curve comprises the steps of: delivering signals R.sub.w, V, Q.sub.0, μ, and φ, corresponding to radius of said borehole, volume of said flowline, rate of fluid flow into said formation testing device, viscosity of said fluid, and porosity of said formation, respectively to said signal processor; determining compressibility of said fluid, and delivering electrical signals C and c correlative thereto; estimating permeability of said formation, and delivering an electrical signal k correlative thereto; determining permeability of said formation and pressure of said formation by altering said electrical signals P, R.sub.w, V, Q.sub.0, μ, φ, C, c, and k according to: ##EQU36## 2. The method of claim 1, further comprising the step of displaying said electrical plot and said electrical type-curve on a monitor.
3. The method as set forth in claim 1, further comprising the step of terminating said testing of said underground formation when said electrical type-curve provides a substantially unchanging estimate of fluid pressure in said underground formation.
4. A method of testing an underground formation, said method comprising the steps of:
disposing a formation testing device within a borehole adjacent a portion of said underground formation to be tested, said formation testing device having a probe for collecting fluid from said formation and having a transducer for measuring fluid pressure, said transducer being fluidically coupled to said probe by a flow line; drawing fluid from said underground formation through said probe and into said formation testing device, and permitting fluid pressure within said formation testing device to build toward fluid pressure within said underground formation; delivering an electrical signal from said transducer to a signal processor electrically coupled to said formation testing device, said electrical signal being correlative to fluid pressure of said fluid in said formation testing device; generating an electrical plot in response to receiving said electrical signal, said electrical plot being correlative to fluid pressure of said fluid in said formation testing device over time; and generating an electrical type-curve that approximates said electrical plot wherein said step of generating an electrical type curve comprises the steps of: delivering signals R.sub.w, V, Q.sub.0, μ, and φ, corresponding to radius of said borehole, volume of said flowline, rate of fluid flow into said formation testing device, viscosity of said fluid, and porosity of said formation, respectively to said signal processor; determining compressibility of said fluid, and delivering electrical signals C and c correlative thereto; estimating permeability of said formation, and delivering an electrical signal k correlative thereto; determining permeability of said formation and pressure of said formation by altering said electrical signals P, R.sub.w, V, Q.sub.0, μ, φ, C, c, and k according to: ##EQU37## 5. A method of testing an underground formation, said method comprising the steps of:
disposing a formation testing device within a borehole adjacent a portion of said underground formation to be tested, said formation testing device having a probe for collecting fluid from said formation and having a transducer for measuring fluid pressure, said transducer being fluidically coupled to said probe by a flow line; drawing fluid from said underground formation through said probe and into said formation testing device, and permitting fluid pressure within said formation testing device to build toward fluid pressure within said underground formation; delivering an electrical signal from said transducer to a signal processor electrically coupled to said formation testing device, said electrical signal being correlative to fluid pressure of said fluid in said formation testing device; generating an electrical plot in response to receiving said electrical signal, said electrical plot being correlative to fluid pressure of said fluid in said formation testing device over time; and generating an electrical type-curve that approximates said electrical plot wherein said step of generating an electrical type curve comprises the steps of: delivering signals R.sub.w, V, Q.sub.0, μ, and φ, corresponding to radius of said borehole, volume of said flowline, rate of fluid flow into said formation testing device, viscosity of said fluid, and porosity of said formation, respectively to said signal processor; determining compressibility of said fluid, and delivering electrical signals C and c correlative thereto; estimating permeability of said formation, and delivering an electrical signal k correlative thereto; determining permeability of said formation and pressure of said formation by altering said electrical signals P, R.sub.w, V, Q.sub.0, μ, φ, C, c, and k according to: ##EQU38## 6. A method of interpreting formation pressure data P electrically recorded by a formation testing device within a borehole adjacent a portion of an underground formation, said formation testing device having a probe for collecting fluid from said formation and having a transducer for measuring fluid pressure, said transducer being fluidically coupled to said probe by a flow line, said method comprising the steps of:
delivering said electrically recorded pressure data P versus time t to a signal processor; delivering electrical signals R.sub.w, V, Q.sub.o, μ, and φ, corresponding to radius of said borehole, volume of said flow line, rate of fluid flow into said formation testing device, viscosity of said fluid, and porosity of said formation, respectively, to said signal processor; said signal processor: determining permeability of said formation and pressure of said formation by altering said electrical signals P, R.sub.w, V, Q.sub.o, μ, φ, C, γ, and k according to: ##EQU39## 7. A method of testing an underground formation, said method comprising the steps of:
drilling a borehole into said underground formation; disposing a formation testing device within said borehole adjacent a portion of said underground formation to be tested, said formation testing device having a probe for collecting fluid from said formation and having a transducer for measuring fluid pressure, said transducer being fluidically coupled to said probe by a flow line; drawing fluid from said underground formation through said probe and into said formation testing device; delivering an electrical signal P from said transducer to a signal processor electrically coupled to said formation testing device, said electrical signal P being correlative to fluid pressure of said fluid in said formation testing device; recording said electrical signal P over time t to generate an electrical plot being correlative to fluid pressure of said fluid in said formation testing device over time; delivering electrical signals R.sub.w, V, Q.sub.o, μ, and φ, corresponding to radius of said borehole, volume of said flow line, rate of fluid flow into said formation testing device, viscosity of said fluid, and porosity of said formation, respectively, to said signal processor; determining permeability of said formation and pressure of said formation by altering said electrical signals P, R.sub.w, V, Q.sub.o, μ, φ, C, and k according to: ##EQU40## Descripción 1. Field of the Invention The present invention relates generally to the field of oil and gas exploration and, more particularly, to a method and apparatus for performing pressure tests in an underground formation containing oil and/or gas. 2. Description of the Related Art Hollywood leads one to believe that the exploration and production of oil and gas is a trivial matter, based largely on luck. One merely erects a derrick on a piece of arid Texas land and drills an oil well. The ensuing gusher creates a festival-like atmosphere among the workers and makes the owner an instant millionaire. However, the exploration and production of oil and gas is serious business. Over the past several decades, those skilled in the art have developed highly sophisticated techniques for finding and producing oil and gas (commonly referred to as "hydrocarbons") from underground formations. These techniques facilitate the discovery of underground hydrocarbon-producing formations and the subsequent assessment and production of such formations. When a formation containing hydrocarbons is discovered, a borehole is drilled into the formation from the surface so that tests may be performed on the formation. Typically, samples of the penetrated formations are tested to determine whether hydrocarbons are indeed present, whether the penetrated formation is similar to nearby formations, and whether the formation is likely to be of commercial value. As part of these preliminary tests, wireline logging tools may be lowered into the borehole to determine various characteristics of the formation, such as the porosity and size of the formation. One such wireline logging tool is generically referred to as a formation tester. Known wireline formation testers are slender tools that are positioned at a depth in a borehole adjacent a location in the formation for which data is desired. After the formation tester is lowered into a borehole via the wireline, it sealingly contacts the borehole wall with a probe or snorkel to collect data from the formation. The formation tester collects samples of formation fluid to determine fluid properties, such as viscosity. The formation tester also measures the fluid pressure of the formation over a selected period of time to determine the permeability of the formation and the fluid pressure in the formation. The type of fluid found in the formation and the permeability and pressure of the formation are important factors in determining the commercial usefulness of the well and the manner in which the fluid should be removed from the well. A formation tester typically performs a pretest sequence that includes a "drawdown" cycle and a "buildup" cycle. During a drawdown cycle, the tester draws in fluid from the formation. To draw the fluid into the tester, a pressure drop is created at the probe by retracting a piston in the tester's pretest chamber. Once the piston stops retracting the drawdown cycle ends and the buildup cycle begins. During the buildup cycle, fluid continues to enter the tester, and the pressure in the tester begins to increase. The fluid continues to enter the tester until the fluid pressure within the tester equals the formation pressure, or until the differential pressure between the tester and the formation becomes insufficient to drive connate fluids into the tester. The operator monitors the pressure at a console while the logging system simultaneously records the pressure data. When the operator determines that the buildup cycle has ended, he begins another drawdown cycle or moves the tester to a different location. The data recorded during the drawdown and buildup cycles may be later interpreted to determine crucial parameters related to the formation, such as fluid pressure in the formation and permeability of the formation. The value of analyzing the pressure response of a formation was recognized by those skilled in the art shortly after World War II. Over the years, talented engineers have continually revised and built upon these pressure analysis techniques in an effort to improve the determination of the characteristics of the formation, such as pressure and permeability. In 1970, these techniques were greatly enhanced by the introduction of type-curve matching techniques, which, simply put, attempt to match the data (or curve) of pressure vs. time measured by the formation tester with a like curve of pressure vs. time determined from a mathematical model of fluid flow. This approximate curve is then used to determine the characteristics of the formation. In fact, in the 25 years since the introduction of type-curve matching, many skilled in the an have concentrated on developing a multitude of approximate curves and analysis techniques to take into account different formation characteristics, such as different geometries, anisotropic porosity, fractures, and boundaries. Traditional techniques for interpreting the pressure data compiled by a formation tester are typically performed on the recorded data after the test has been completed. Although the interpretation may be performed at the well site, the pressure test is terminated and all testing suspended in order to interpret the data. Typical type-curve matching requires that the actual pressure change vs. time be plotted in any convenient units on log-log tracing paper, using the same scale as the type curve. Then, points plotted on the tracing paper are placed over the type curve. Keeping the two coordinate axes parallel, the measured curve is shifted to a position on the type curve that represents the best fit of the measurements. To evaluate reservoir constants, a match point is selected anywhere on the overlapping portion of the curves, and the coordinates of the common point on both sheets of paper are recorded. Once the match is obtained, the coordinates of the match point are used to compute formation flow capacity, kh, and storativity-thickness, φc.sub.t h. Not only are these traditional type-curve matching techniques laborious, the type-curve matching does not necessarily render accurate information. Many factors influence the accuracy of the measured pressure data and of the interpretation techniques. For instance, the internal volume of known wireline formation testing tools can act as a fluid "cushion," which tends to alter measured data from theoretically ideal data. Thus, this cushioning effect leads to significant errors in the rates of drawdown and buildup detected by such tools, resulting in unreliable estimates of important parameters of earth formations. These errors are due primarily to the compressibility of the fluid contained in the tester's flow lines and chambers. Such compressibility, referred to herein as the "flow line storage effect," generally slows the rate of pressure drawdown and buildup. In subsequent analysis of the measured data, it is very difficult to distinguish between pressure changes resulting from the formation and those due to flow line storage effects. Ultimately, the flow line storage effects create serious problems in data interpretation and can lead to large errors in estimated characteristics of the formation, such as permeability. Thus, traditional techniques use "late time data", i.e., data collected near the end of the buildup cycle, to estimate permeability and pressure, because the flow line storage effects distort the early time data and make it unusable by these techniques. Using these techniques, radial time, spherical time, and derivative plots are used to select a small portion of late time data to fit a straight line. The slope of the line determines the permeability of the formation, and the intercept determines the pressure. As a result, most of the recorded pressure data is unusable. Also, the buildup cycle cannot be terminated until the buildup pressure substantially reaches the formation pressure. In high permeability formations, continuing the buildup cycle until the buildup pressure substantially reaches the formation pressure poses few problems. The drawdown and buildup cycles usually require a short period of time, typically about five minutes. After the desired measurements are made, the formation tester may be raised or lowered to a different depth within the formation to take another series of tests. However, formations having low permeabilities in the range of 0.001 to 1 millidarcy, commonly referred to as "tight zones," require a considerably greater time for the buildup pressure to occur, often hours and sometimes days. Thus, because the operator cannot terminate the buildup cycle until the measured pressure substantially reaches the formation pressure, testing may take considerable time. Also, tight zones tend to magnify the effects of flow line distortion. Although some more recent methods are capable of interpreting early time data, these methods require numerical rather than analytical solutions. For example, some numerical solutions include storage effects, but the results are presented in voluminous plotted charts; this impedes the interpretation process, which is also disadvantaged by the inability to interpolate results to parameters not plotted. Such numerical solutions are not amenable to simple physical interpretation, and, thus, cannot be used in real time, i.e., during the formation test, to facilitate the testing. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. In accordance with one aspect of the present invention, there is provided a method of testing an underground formation. The method may include the following steps. A formation testing device is disposed within a borehole adjacent a portion of the underground formation to be tested. The formation testing device includes a probe for collecting fluid from the formation and a transducer for measuring fluid pressure. The transducer is fluidically coupled to the probe by a flow line. Fluid is drawn from the underground formation through the probe and into the formation testing device. The fluid pressure within the formation testing device is permitted to build toward the pressure of fluid within the underground formation. An electrical signal from the transducer is delivered to a signal processor that is electrically coupled to the formation testing device. The electrical signal is correlative to fluid pressure of the fluid in the formation testing device. An electrical plot is generated in response to receiving the electrical signal. The electrical plot is correlative to fluid pressure of the fluid in the formation testing device over time. An electrical type-curve is generated that approximates the electrical plot. Additionally, the method may include the following steps. The electrical plot and the electrical type-curve may be displayed on a monitor. Also, the testing of the underground formation may be terminated when the electrical type-curve provides a substantially unchanging estimate of fluid pressure in the underground formation. In accordance with another aspect of the present invention, there is provided a method of testing an underground formation. The method may include the following steps. A borehole is drilled into the underground formation. A formation testing device is disposed within a borehole adjacent a portion of the underground formation to be tested. The formation testing device includes a probe for collecting fluid from the formation and a transducer for measuring fluid pressure. The transducer is fluidically coupled to the probe by a flow line. Fluid is drawn from the underground formation through the probe and into the formation testing device. An electrical signal P from the transducer is delivered to a signal processor that is electrically coupled to the formation testing device. The electrical signal P is correlative to fluid pressure of fluid in the formation testing device. The electrical signal P is recorded over time t to generate an electrical plot that is correlative to fluid pressure of the fluid in the formation testing device over time. Electrical signals R.sub.w, V, Q.sub.o, μ, and φ, corresponding to radius of the borehole, volume of the flow line, rate of fluid flow into the formation testing device, viscosity of the fluid, and porosity of the formation, respectively, are delivered to the signal processor. The compressibility of the fluid in the flowline C and the compressibility of the formation fluid c are determined, and correlative electrical signals C and c are delivered. The permeability of the formation is estimated, and a correlative electrical signal k is delivered. The permeability of the formation and pressure of the formation are determined by altering the electrical signals P, R.sub.w, V, Q.sub.o, μ, φ, C, c, and k according to: ##EQU1## and c is approximately equal to C. Turning now to the drawings and referring initially to FIG. 1, an exemplary wireline formation tester 10 is schematically depicted as deployed in an uncased ("open hole") well borehole 12. Although the preferred analysis method described herein may be utilized with a variety of formation testers, a preferred formation tester is a model SFFT-B tool available from Halliburton Logging Services, Inc. The tester 10 is suspended on a wireline 14. At the earth's surface 16, the wireline 14 passes over a sheave 18 before entering the borehole 12 and is stored on a drum 20. The wireline 14 is operatively coupled to a central processing unit ("CPU") 22 which processes data communicated from the tester 10 via the wireline 14. One preferred processor is an XL2000 real time computer used by Halliburton Logging Services, Inc., but those skilled in the art will readily recognize suitable alternatives. One preferred processor is an IBM 7006 Graphics Work Station 41T. This IBM work station uses an 80 megahertz power PC 601 processor with an optional L2 cache. It further includes a graphics adaptor, 540 megabyte disk drive, and 16 megabyte memory. The wireline 14 preferably includes a data communication line coupled to a recording unit 26 that records the depth of penetration of the tester 10 in the borehole 12, by known techniques. An operator (not shown) controls the formation tester 10 via a console and monitor (not shown) operatively associated with the CPU 22, as is well known in the art. The borehole 12 contains well fluid 30, which is typically a combination of drilling fluid that has been pumped into the borehole 12 and connate fluid that has seeped into the borehole 12 from the formation 28. The drilling fluid may be water or a water-based or oil-based drilling fluid. The density of the drilling fluid is usually increased by adding certain types of solids, such as barite and other viscosifiers, that are suspended in solution. Such fluids are often referred to as "drilling muds." The solids increase the hydrostatic pressure of the well fluid 30 to help maintain the well and keep fluids from surrounding formations from flowing into the well. The solids within the drilling fluid create a "mudcake" 33 as they flow into the formation 28 by depositing solids on the inner wall of the borehole 12. Conventional mudcakes are typically between about 0.25 and 0.5 inch thick, and polymeric mudcakes are often about 0.1 inch thick. The wall of the borehole 12, along with the mudcake 33 of deposited solids, tends to act like a filter. The mudcake 33 also helps prevent excessive loss of drilling fluid into the formation 28. The fluid pressure in the borehole 12 and the surrounding formation 28 is typically referred to as "hydrostatic pressure." Relative to the hydrostatic pressure in the borehole 12, the hydrostatic pressure in the mudcake 33 decreases rapidly with increasing radial distance. Pressure in the formation 28 beyond the mudcake 33 gradually tapers off with increasing radial distance outward from the wellbore. Once the wireline formation tester 10 is deployed in the borehole 12 adjacent an earth formation 28 of interest, the tester 10 may be used to collect pressure data concerning the earth formation 28, to collect samples of connate fluids within the formation, and to gather other data on downhole conditions. Since such tests are generally performed in the presence of the well fluid 30 that fills the borehole 12 to a certain depth, the formation tester 10 is preferably constructed within an elongated, sealed sonde 32 of sufficient strength to withstand hydrostatic pressures prevailing in the borehole 12 at the depths of the formations of interest. Considering the formation tester 10 in greater detail, still referring to FIG. 1, the formation tester 10 includes a laterally extendable probe 34 surrounded by an isolation packer 36. The tester 10 also includes one or more back-up pads or shoes 38 and 39 preferably arranged to extend laterally in a direction diametrically opposed to the probe 34. The extended pads 38 and 39 stabilize the tester 10 and ensure proper sealing of the isolation packer 36 against the mudcake 33 of the borehole 12 during test periods. The probe 34, the isolation packer 36, and the back-up pads 38 and 39 are advantageously arranged to extend laterally from the sonde 32 when the tester 10 is properly positioned adjacent a formation 28 of interest, as known to those skilled in the art. Internally, the tester 10 includes an equalization valve 48, a pressure sensor 50, and a pretest chamber 54. The tester 10 may also include one or more fluid storage tanks 56 and 58 to collect connate fluids from the earth formation 28. Other components may also be advantageously provided in the tester 10, such as instruments for measuring flow rate, temperature, conductivity, and so on, as generally known to those skilled in the art. However, while an exemplary arrangement of the elements of the tester 10 is described herein, various other physical configurations may be used. The equalization valve 48, the pressure sensor 50, the probe 34, the pretest chamber 54, and the storage tanks 56, 58 are all interconnected by a flow line 64 of predetermined, or known, volume. Although the particular dimensions and positions of the primary components of the tester 10 will influence the volume of the flow line 64, this volume will be known when the particular tester 10 is constructed. The equalization valve 48 selectively enables fluid communication between the well fluid 30 and the flow line 64. Such communication allows the probe 34 to seal properly against the formation 28 prior to testing and facilitates the retraction of the probe 34 at the end of the test. The equalization valve 48 is capable of remote actuation by the hydraulic system 46 or an electrical system, such as solenoid or motor (not shown), in response to a signal transmitted from the CPU 22 through the wireline 14. The pressure sensor 50 preferably comprises a transducer capable of generating an electrical output signal proportional to fluid pressure, such as a quartz pressure transducer. The pressure sensor 50 may be used to detect and measure pressures near the probe 34 during the drawdown and buildup phases of testing, as will be described. Although the sensor 50 is shown schematically at some distance from the probe 34, the location of the sensor in an actual physical embodiment is preferably as close to the probe 34 as practically possible. This placement of the sensor 50 ensures accurate readings of the pressures caused by fluid flow from the earth formation 28. The present chamber 54 includes a piston 66, a cylinder 68, and a hydraulically operated actuator 69 responsive to the hydraulic system 46 and to control signals transmitted through the wireline 14 from the CPU 22. As described in greater detail below, the retraction of the piston 66 in the test chamber 54 decreases the pressure in the tester 10 during a drawdown phase of a formation test, thereby increasing the volume of the chamber 54 in communication with the flow line 64. The number and volume of the chamber(s) 54 and of storage tanks 56 and 58 may be selected depending upon the test data to be collected and upon the number and volume of fluid samples to be collected. The valves 60 and 62 associated with the storage tanks 56 and 58 respectively may be electrically or hydraulically operated and are responsive to control signals from the CPU 22 transmitted through the wireline 14. Various drawdown/buildup pressure curves 80, 82, and 84 are illustrated in FIG. 2 to show the manner in which flow line storage effects influence the pressure measurements taken by a traditional formation tester. The curve 80 represents an ideal drawdown/buildup pressure vs. time curve representative of data taken by a formation tester having no flow line storage effect. In this ideal case, the pressure at the probe drops very rapidly from the prevailing well fluid pressure of 12,000 pounds per square inch (psi) upon initiation of the drawdown phase, and the pressure rises rapidly to the formation pressure during the buildup phase. By contrast, the curve 82 represents the flow line storage effect's influence on data taken by a formation tester having a flow line storage capacity of 105 cc and a pretest chamber volume of 5 cc, where the flow rate into the probe from the formation is 0.5 cc/see. As can be seen, the pressure drops less and much more slowly during the drawdown phase, and the pressure rises much more slowly during the buildup phase. Finally, the curve 84 represents the flow line storage effect 's influence on data taken by a formation tester having a flow line storage capacity of 120 cc and a pretest chamber volume of 20 cc, where the flow rate into the probe from the formation is 2.0 cc/sec. Here, the pressure at the probe drops rapidly, though not as rapidly as the ideal case, during the drawdown phase, but rises much more slowly during the buildup phase. In the illustrative embodiment, the wireline formation tester 10 may be operated as follows. The sonde 32 is lowered within the borehole 12 to a depth corresponding to the location of an earth formation 28 from which data is desired. The sonde's depth of penetration is indicated by a counter (not shown) associated with the sheave 18 and recorded by the recording unit 26. During this deployment period, the equalization valve 48 is open to equalize pressures within the sonde 32 with the hydrostatic pressure of the surrounding well fluid 30. With the sonde positioned adjacent the formation 28, the CPU 22 transmits signals via the wireline 14 causing the equalization valve 48 to close and causing the back-up pads 38 and 39 and the probe 34 to extend and contact the formation 28. Initially, as the isolation packer 36 is extended against the borehole wall, the pressure inside the probe 34 slightly increases, as shown beginning at t.sub.set by the pressure vs. time curve 100 in FIG. 3. This pressure increase followed by a decrease is illustrated in FIG. 3 by the curve portion 102 prior to the start of the pretest. After the probe 34 is in firm contact with the formation 28, and after a seal is established by the isolation packer 36, the desired formation test sequence may begin. It should be appreciated that the isolation packer 36 helps prevent the well fluid 30 from flowing outward through the mudcake 33 and circling back into the probe 34 and the pretest chamber 54. Thus, the isolation packer 36 "isolates" the probe 34 from the well fluids 30 in the borehole 12, helping to ensure that the measurements of the probe 34 are representative of the pressure of the connate fluid in the formation 28. An exemplary drawdown phase of the formation test may proceed as follows. Formation fluid is drawn into the tester 10 by decreasing the pressure in the tester 10, as shown by the curve portion 104 beginning at t.sub.start. The pressure is reduced by retracting the piston 66 within the cylinder 68, thus expanding the test chamber 54. When the pressure within the tester 10 has been sufficiently reduced to the drawdown pressure P.sub.dd, the pretest piston is stopped 54 causing the buildup phase to begin. During the build-up phase, the reduced pressure in the tester 10 in the vicinity of the probe 34 continues to draw connate fluids or mud filtrate from the formation 28 into the tester 10 through the probe 34. As these fluids enter and fill the tester 10, the pressure detected by the sensor 50, as shown by the curve portion 106, rises to p.sub.bu which approaches equilibrium with the formation pressure. This final buildup pressure p.sub.bu is frequently referred to as the "sandface pressure." It is usually assumed that the sandface pressure is close to the formation pressure. This equilibrium marks the close of the buildup phase of the test. When the formation tester 10 is disengaged from the borehole wail at t.sub.bu, the detected formation pressure increases rapidly from p.sub.bu, as shown by the curve portion 108, due to the removal of pressure applied by the isolation packer 36. During both the drawdown and buildup phases, the pressure sensor 50 measures the pressure prevailing near the probe 34 and transmits output signals corresponding to this pressure to the CPU 22 through the wireline 14. The CPU 22 in turn causes the pressure readings to be stored in the recording unit 26, along with the time at which the readings were taken, the depth of the formation 28, and other data produced by the tester 10. When the formation test is complete, a sample of the fluids may be stored in one or more of the storage tanks 56 or 58 by opening the valve 60 or 62. When this operation is complete, the equalization valve 48 is opened and the probe 34 and back-up pads 38 and 39 are retracted. The tester 10 may then be repositioned at another depth, or it may be removed from the borehole 12. The drawdown and buildup pressures p.sub.dd and p.sub.bu are used in determining formation permeability. The rate of the pressure buildup to p.sub.bu is slowed, however, primarily due to the cushion effect of the flowline 64 volume, which is generally greater than the volume of pretest chamber 54. This flowline cushion effect renders much of the buildup cycle unusable for known pressure/flow analysis techniques, such as the radial or "Horner" analysis or spherical models. This flowline distortion in the buildup pressure does not dissipate until the difference in the recorded pressure and the final buildup pressure is small. Although FIG. 3 illustrates a pretest sequence having a single drawdown and buildup phase, the formation test sequence may include various drawdown and/or buildup phases. For permeable zones, i.e., zones having permeabilities from 1 to 1000 millidarcies, the formation fluid production rate at the probe is approximately the volume rate of the pretest piston. The pressure drops rapidly during the first few seconds of the pretest and then stabilizes to a nearly constant drawdown value for the remainder of the pretest. The buildup is also very rapid, with the most dynamic changes occurring during the first few seconds of the buildup. FIG. 4 illustrates this type of pretest behavior. FIG. 4 also illustrates the practice of taking multiple pretests while maintaining the packer seal. The first pretest is usually shorter and creates a greater pressure drop than the second pretest. When the formation tester is set against the formation, the probe traps the mudcake, which is removed during the first pretest. The process of removing mudcake and cleaning the area around the probe tends to distort the pressure curve. This makes the second pretest preferable to use for drawdown and buildup pressure analysis. Multiple pretests for tight zones are usually not practical, however, due to the long buildup times. In tight zones, the rate of pressure buildup is slowed, primarily due to the flow line storage effect. Because the flow line storage effect can last for most of the buildup time, this portion of the pressure vs. time data is not suitable for a Horner type analysis. The distortion in the pressure vs. time data due to flow line storage effects does not dissipate until the difference in the recorded pressure and the final buildup pressure is small. It may be difficult to identify this distortion on a Horner type plot, and, if this portion of the data is used, the error in the calculated permeability can be large. The traditional interpretation models used to solve for formation properties do not account for the volume of fluid in the tool in contact with the formation which is usually referred to as the flow line volume. In practice, if this is not considered, the pressure drawdown and buildup curves do not follow the expected buildup or decline models until the effects of flow line storage have dissipated. This can take considerable time for many test conditions and is difficult to recognize in the pressure data. The new method uses a closed form solution developed for spherical flow satisfying exact boundary conditions. It should be noted that all currently known models are presented in Laplace space and inverted numerically to provide timewise behavior because a closed form solution could not be determined. While the Laplace space formulation has been used to generate solutions for spherical flow, its usefulness is limited by inaccurate numerical techniques and cumbersome presentation of computed results. The exact closed form pressure solution of spherical flow with storage effects and assuming a continuously acting constant Q.sub.0 is given in equations 1 and 3 (see Appendix I for derivation). By using direct curve matching techniques, the drawdown and buildup pressures described by equations 1 and 2 are used to find formation properties from wireline formation pressure tests. The physical pressure at the effective probe or well radius (R.sub.w) is: ##EQU2## As indicated above, Equation 1 is the solution for the drawdown stage of the test and assumes a constant rate (Q.sub.0 greater than 0) drawdown only, beginning at time t=0 and lasting indefinitely. In practice, production is shut-in after a time t.sub.pro, and no fluids are produced. To represent the complete drawdown and buildup process, Equation 2, which is obtained by linear superposition, is used. ##EQU3## where the dimensionless pressure-time relationship is given by: ##EQU4## The complex constants β.sub.1 and β.sub.2 satisfy: ##EQU5## and the dimensionless radius and time is given by: ##EQU6## The "bore hole shape factor" γ has been added to the exact solution developed in Appendix I to account for non-spherical cylindrical borehole boundary effects (see Equation 6). The solution developed in Appendix I is for a perfect spherical problem. The borehole represents an interruption to sphericity and can be accounted for with the shape factor introduced in Equation 6. The shape factor is determined using full three-dimensional finite difference or finite element modeling methods. The specific dimensional effects of a cylindrical borehole is modeled in an infinite formation and the shape factor is determined by comparing the exact solution (Equations 1 and 2) to the modeled solution. If the borehole radius is zero there are no borehole effects and γ=0 yields the exact solution for spherical flow given in Appendix I. For a borehole of infinite radius the shape factor is γ=1 which is the exact solution for hemispherical flow. In practice the borehole is large with respect to the formation tester probe radius, and the flow factor is nearly 1. The constants in the equations are: R.sub.w =radius of well or probe of production (cm) P.sub.o =initial and formation pressure (atm) P(R.sub.w,t)=pressure at the probe (atm) t=test time (sec) r.sub.w =dimensionless radius (nondimensional) p(r.sub.w,t)=dimensionless pressure (nondimensional) t=dimensionless time (nondimensional) V=volume of the flow line (cc) C=compressibility of the flow line fluid (1/atm) c=compressibility of the formation fluid (1/atm) γ=borehole hemispherical shape factor Q.sub.o =injection or production flow rate (cc/sec) μ=viscosity of formation fluid (cp) k=permeability of formation (darcy) φ=porosity of formation (nondimensional) While the primary constants determined from traditional models are the permeability k and initial formation pressure P.sub.o, equation 1 can be used to solve for additional properties such as compressibility of the flow line fluid and formation fluid, C and c, respectively. However, in practice c and C are approximately equal and simplifies the determination of compressibility. An approximation to equation 2 can also be used (See Appendix I for derivation): ##EQU7## Equation 8 has been found to closely match the exact solution given in equation 3 over the operating range of t and r.sub.w of interest to wireline formation testers. Furthermore, equation 9 provided below may be used in situations where it cannot be assumed that the flow rate Q.sub.o is constant. Instead, time dependent rates F(t) may be considered, where equation 9 reduces to equation 1 when F(t)=1 (see Appendix I for derivation). ##EQU8## By using equation 2 with equations 3, 8, or 9 to match pressure data from formation pressure tests, a more precise technique has been developed to determine formation properties such as formation pressure and permeability. Because the entire pressure time history is matched, rather than only a small late time segment, it is unnecessary for the person making the estimates to determine what portion of the data is to be used since virtually all the data is used for curve matching. Because all of the pressure time history is used in the curve matching technique, the speed that formation properties are determined can be increased and the test time shortened. The flowchart 150 illustrated in FIG. 5 describes an electronically implemented method for determining formation properties using the pressure plots obtained by the formation tester. While the flowchart 150 is provided and described for ease of illustration, it should be understood that it is based on an electronic implementation, such as the computer program listed in Appendix II which is loaded into the memory of a suitable processor and executed as is well known by those skilled in the art. Of the constants listed above, the radius of the well R.sub.w, the volume of the flow line V, the injection of production flow rate Q.sub.0, the viscosity of the formation fluid μ, and the porosity of the formation φ are known parameters. In the block 152, these parameters are input into the program. In the block 154, the program receives the pressure data being measured by the formation tester. As illustrated, the drawdown data is read first, and the beginning drawdown cycle t.sub.start and the beginning of the buildup cycle t.sub.dd are determined. Thus, referring to the constants listed above, the pressure at the probe P(R.sub.w,t) and the test time t are measured by the formation tester and utilized by the program. In the block 156, the compressibility of the flow line fluid C and the compressibility of the formation fluid c, which are normally assumed to be the same, are determined. As stated in the flowchart, the program calls the subroutine CALCT (CT) as given in the program listing attached hereto as Appendix II. During the initial drawdown time period, the fluid in the flowline is decompressed by the pretest piston movement. When the drawdown pressure drops below the sandface pressure, the mudcake at the probe is pulled away by the sudden start of fluid being extracted from the formation (assuming a permeable zone). Since the volume of the fluid in the flowline is known (V.sub.fl) and the rate of decompression (Q.sub.0) is known, the compressibility of the flowline fluid can be determined by comparing the pressure derivative to the rate of volume change created by the pretest chamber. The flowline fluid compressibility can be determined by locating the minimum (negative value) of the pressure derivative from the time period t.sub.start to t.sub.dd. The discrete pressure time derivative is defined as follows: ##EQU9## The index of the minimum pressure derivative n=n* is determined during the drawdown time period: ##EQU10## The flowline fluid compressibility can be estimated as follows: ##EQU11## It should be noted that C is recorded on the first minimum pressure derivative. This is because the most accurate estimate of compressibility occurs just prior to the likely removal of the mudcake by the probe. This is also confirmed by equation A-40 in Appendix I. In block 158, an initial estimate of the permeability of the formation k is determined. As illustrated, a subroutine KDDPERM is called, and the estimate of the permeability k is determined, where k=kdd/10. By referring to the subroutine KDDPERM described in the attached Appendix II, it can be seen that if the pretest were to continue for an extended time (i.e., t.sub.pro →∞) equation 1 is used to determine the steady state drawdown pressure. ##EQU12## And the drawdown permeability K.sub.DD can be estimated by solving for K. ##EQU13## An initial estimate of the permeability k is provided because the formation pressure P.sub.o and the permeability of the formation k are determined iteratively using regression analysis. To begin this iterative analysis, a counter N is set to zero in the block 160. In the block 162, the program reads the buildup pressures being measured by the formation tester. The counter N is incremented by M, where M represents a block of data read. The time T, which begins at time t.sub.bu, is incremented by the counter times the change in time Δt between measurements. In block 164, the program performs a chi-square regression analysis by calling the subroutine GRADLS, set forth in Appendix II attached hereto. With the help of the functions FCHISQ and SPHER, the subroutine GRADLS, using equation 1, 7, or 8 (in this case equation 7) determines the formation pressure P.sub.0 and the permeability k, as set forth in the block 166. In the block 168, the pressure data being measured by the formation tester is plotted on a monitor (not shown), along with the calculated curve fit. As will be described in more detail in reference to the later figures, the calculated curve fit provides a projection which estimates future pressure readings. Thus, as the buildup cycle progresses, the calculated curve fit becomes more and more accurate. When the curve fit ceases to change in any meaningful manner as more data points are collected from the measured pressures, the formation test may be stopped because the parameters of interest have been determined accurately by the curve fit. The operator may stop the test by a determination he makes by viewing the plotted curve vs. the pressure data, or the program may make the determination and signal the operator to terminate the test. However, it is possible that the parameters input in the block 152 or the parameters subsequently determined or estimated in the blocks 154, 156, or 158 are inaccurate in some regard, causing the calculated curve fit to deviate significantly from the plot of measured pressure data. If this is the case, parameters may be changed in the block 170. Once changed, the steps described in the blocks 164, 166, and 168 are repeated with the new parameters. If the parameters are not changed, control of the program is passed to the block 172 where the program inquires to whether the test has been terminated at t.sub.stop. Thus, while the analysis proceeds, the block 172 transfers control back to the block 162 to perform another iteration. Once the analysis is complete, control transfers to the block 174 where the program ends. The term t.sub.stop, generally refers to a signal input by the operator indicating that the analysis of the pressure data is complete. In other words, the following scenarios may exist. First, the analysis may take place real time, with the calculated curve fit being plotted against the measured pressure data. Once the operator or program determines that the calculated curve accurately depicts the parameters of the formation, the analysis may be terminated although the pressure test may continue. Second, the operator may terminate the pressure test, at which time the analysis would terminate accordingly. Third, the analysis may be performed on pressure data prerecorded from previous pressure tests. In this case, the analysis would terminate at the end of the pressure test or when the program determines that the curve fit accurately depicts the parameters of the formation before the termination of the pressure test. FIGS. 6, 10, 14, and 18 illustrate four exemplary pretest sequences. The data plot 200 illustrated in FIG. 6 shows an example of a pretest sequence performed in a high permeability formation. The first drawdown/buildup cycle is primarily used for clearing the mudcake from the probe. The second drawdown/buildup cycle is analyzed to determine characteristics of the formation. The curve portion 202 illustrates the hydrostatic pressure in the borehole while the formation tester is being positioned. At about t.sub.set, the pressure rise illustrated by the curve portion 204 indicates that the probe has been placed against the wall of the borehole. At t.sub.start, the first drawdown cycle begins, as illustrated by the curve portion 206. At t.sub.dd1, the first buildup cycle begins, as illustrated by the curve portion 208. It should be noticed that the first buildup cycle is prematurely terminated before the final buildup pressure is permitted to stabilize, primarily because the first drawdown/buildup cycle is intended to clear the mudcake from the borehole wall to facilitate a second, more accurate, drawdown/buildup cycle. This second drawdown cycle begins at t.sub.start2, as illustrated by the curve portion 210. The second buildup cycle begins at t.sub.start2, as illustrated by the curve portions 212 and 213. As can be seen, the pressure is allowed to build until the measured pressure stabilizes at p.sub.bu. At t.sub.end, the probe is removed from the wall of the borehole, as indicated by the curve portion 214. The second drawdown/buildup cycle is illustrated in greater detail in FIG. 7, as indicated by the curve portions 210, 212, and 213. Using conventional type-curve matching techniques, the end portion of the curve portion 213 is plotted as shown by the curve portion 216 in FIG. 8, and a straight line 218 is curve-fitted to a linear portion of the curve portion 216. Thus, it can be seen that only a very small portion of the total data collected in the plot 200 is actually used to determine the permeability and formation pressure of the formation. However, as illustrated in FIG. 9, using the new technique described herein, the CPId generates a type curve 220 which matches the entire second drawdown/buildup cycle, thus making all of the data making up the curve portions 210, 212, and 213 usable in determining the characteristics of the formation. The data plot 230 illustrated in FIG. 10 shows an example of a pretest sequence performed in a low permeability formation. In a low permeability formation, a drawdown/buildup cycle may take over an hour before the measured pressure stabilizes at p.sub.bu. Therefore, a first drawdown/buildup cycle is typically not performed. Thus, even though the mudcake may affect the measured pressure, the first, and only, drawdown/buildup cycle is analyzed to determine characteristics of the formation. As shown in FIG. 10, the curve portion 232 illustrates the hydrostatic pressure in the borehole while the formation tester is being positioned. At about t.sub.set, the pressure rise illustrated by the curve portion 234 indicates that the probe has been placed against the wall of the borehole. At t.sub.start, the drawdown cycle begins, as illustrated by the curve portion 236. At t.sub.dd, the buildup cycle begins, as illustrated by the curve portion 238. As can be seen, the pressure is allowed to build until the measured pressure levels at p.sub.bu. At t.sub.end, the probe is removed from the wall of the borehole, as indicated by the curve portion 240. A comparison of the plot 230 with the plot 200 shows that the pressure drops more slowly and rises more slowly in the low permeability formation as compared with a high permeability formation. The drawdown/buildup cycle is illustrated in greater detail in FIG. 11, as indicated by the curve portions 236 and 238. Using conventional type-curve matching techniques, the end portion of the curve portion 238 is plotted as shown by the curve portion 242 in FIG. 12, and a straight line 244 is curve-fitted to a linear portion of the curve portion 242. Thus, it can be seen that only a very small portion of the total data collected in the plot 230 is actually used to determine the permeability and formation pressure of the formation. However, as illustrated in FIG. 13, using the new technique described herein, the CPU generates a type curve 246 which matches the entire drawdown/buildup cycle, thus making all of the data making up the curve portions 236 and 238 usable in determining the characteristics of the formation. Because the new technique makes use of all of the measured data, the pressure during the buildup cycle need not stabilize at the formation pressure. All that is generally recommended is that the pressure be measured in the buildup cycle until sufficient data is acquired to determine an accurate curve fit. Thus, the pretest sequence may be substantially shortened or revised. For instance, as illustrated by the plot 260 in FIG. 14, several drawdown/buildup cycles may be performed in low permeability formations in the time previously allocated to a single drawdown/buildup cycle. The first drawdown/buildup cycle is primarily used for clearing the mudcake from the probe. The curve portion 262 illustrates the hydrostatic pressure in the borehole while the formation tester is being positioned. At about t.sub.set, the pressure rise illustrated by the curve portion 264 indicates that the probe has been placed against the wall of the borehole. At t.sub.start, the first drawdown cycle begins, as illustrated by the curve portion 266. At t.sub.dd1, the first buildup cycle begins, as illustrated by the curve portion 268. It should be noticed that the first buildup cycle is prematurely terminated before the final buildup pressure is permitted to stabilize, primarily because the first drawdown/buildup cycle is intended to clear the mudcake from the borehole wall to facilitate a second, more accurate, drawdown/buildup cycle. The second drawdown cycle begins at t.sub.start2, as illustrated by the curve portion 270. However, as can be seen from a study of the curve portion 270, the pressure fluctuates during the drawdown cycle. This fluctuation may indicate that the tester is partially clogged. The second buildup cycle begins at t.sub.dd2, as illustrated by the curve portion 272. The pressure builds until the measured pressure begins to level off at the formation pressure. However, because of the uncertainty of the second cycle, the operator chooses to perform a third drawdown/buildup cycle, which begins at t.sub.start3, as illustrated by the curve portion 274. The third drawdown cycle appears much smoother than the second drawdown cycle, indicating that the blockage of the tester has been cleared. The third buildup cycle begins at t.sub.dd3, as indicated by the curve portion 276. Again, the pressure builds until the measured pressure begins to level off at p.sub.bu. At t.sub.end, the probe is removed from the wall of the borehole, as indicated by the curve portion 278. Subsequent evaluation indicates that the second drawdown/buildup cycle is analyzed according to the new technique to provide accurate formation information, while analysis using conventional techniques provides inaccurate information. The second drawdown/buildup cycle is illustrated in greater detail in FIG. 15, as indicated by the curve portions 270 and 272. Using conventional type-curve matching techniques, the end portion of the curve portion 272 is plotted as shown by the curve portion 279 in FIG. 16, and a straight line 280 is curve-fitted to a linear portion of the curve portion 279. However, it can be seen that the curve portion 279 contains two linear portions, 282 and 284. The straight line 280 is curve-fitted to the linear portion 284, but it could just as easily be curve-fitted to the linear portion 282. Obviously, the linear portion 282 or 284 to which the straight line 280 is fitted will greatly affect the information provided by the second buildup cycle. However, as illustrated in FIG. 17, using the new technique described herein, the CPU generates a type curve 290 which matches the entire second drawdown/buildup cycle, thus making all of the data making up the curve portions 270 and 272 usable in determining the characteristics of the formation. Thus, the fluctuation in the pressure measured toward the end of the second buildup cycle barely affects the accuracy of the information provided by the type curve 290. As previously mentioned, all that is generally recommended is that the pressure be measured in the buildup cycle after sufficient data is acquired to determine an accurate curve fit. Thus, the buildup cycle may be terminated before the pressure even begins to stabilize near the formation pressure, thus further shortening the pretest sequence. Such a pretest sequence is illustrated by the plot 300 shown in FIG. 18. Although only a single drawdown/buildup cycle need be performed, FIG. 18 illustrates two such cycles, where the first cycle is primarily intended to clear the mudcake from the probe. The second drawdown/buildup cycle is analyzed to determine characteristics of the formation. The curve portion 302 illustrates the hydrostatic pressure in the borehole while the formation tester is being positioned. At about t.sub.set, the pressure rise illustrated by the curve portion 304 indicates that the probe has been placed against the wall of the borehole. At t.sub.start, the first drawdown cycle begins, as illustrated by the curve portion 306. At t.sub.dd1, the first buildup cycle begins, as illustrated by the curve portion 308. The first buildup cycle is prematurely terminated before the final buildup pressure is permitted to stabilize, primarily because the first drawdown/buildup cycle is intended to clear the mudcake from the borehole wall to facilitate a second, more accurate, drawdown/buildup cycle. This second drawdown cycle begins at t.sub.start2, as illustrated by the curve portion 310. The second buildup cycle begins at t.sub.dd2, as illustrated by the curve portion 312. As with the first buildup cycle, the second buildup cycle is terminated before the final buildup pressure is permitted to stabilize in order to shorten testing time. At t.sub.end, the probe is removed from the wall of the borehole, as indicated by the curve portion 314. The second drawdown/buildup cycle is illustrated in greater detail in FIG. 19, as indicated by the curve portion 310 and 312. Conventional type-curve matching techniques cannot be used on such data because buildup pressure has not been allowed to reach formation pressure. Thus, any straight line curve-fitter to the data as previously shown would render very inaccurate information. As illustrated in FIG. 20, using the new technique described herein, the CPU generates a type curve 320 which matches the entire second drawdown/buildup cycle and which predicts the ultimate formation pressure, as shown by the portion of the type curve 320 which extends past the last measured pressure data. The present invention thus provides for an exact spherical flow model which considers the effects of flowline storage for use with formation testers. The generation of the type curve is applicable to accumulated pressure data and may be used to predict formation pressure prior to achieving near steady state values near the formation pressure. Various changes may be made to the disclosed embodiment and inventive concept without departing from the spirit of the claimed invention. Let us consider transient, compressible, liquid Darcy flow in a homogeneous, isotropic medium, and specifically study the spherically symmetric flow produced into a "spherical well" of radius R.sub.w from an infinite reservoir. Let P(r,t) represent the fluid pressure, where r and t are radial and time coordinates. Also, let P.sub.0 denote the constant initial and farfield pressure, while φ, μ, c, and k, respectively, refer to rock porosity, fluid viscosity, combined rock-matrix and fluid compressibility, and formation permeability. In addition, let V denote the volume associated with the storage capacity of the spherical well, e.g., the flow line volume in the formation tester, with C being the compressibility associated with this volume. Finally, we denote by Q(t) the total volume rate produced by the spherical well, due to sandface production plus volume storage effects. The boundary value problem is completely specified by the mathematical model
∂.sup.2 P(r,t)/∂r.sup.2 +2/r∂P/∂r=(φμc/k)∂P/.differential.t (A-1)
P(r,t=0)=P.sub.0 (A- 2)
P(r=∞,t)=P.sub.0 (A- 3)
(4 πR.sub.w.sup.2 k/μ)∂P(R.sub.w,t)/∂r-VC∂P/.differential.t=Q(t) (A-4) Problem formulation. In order to introduce analytical simplifications and to determine the governing dimensionless parameters in their most fundamental form, we define the nondimensional italicized variables r, t, and p, which are respectively normalized by the quantities r*, t*, and p*, as follows,
r=r/r* (A-5)
t=t/t* (A-6)
p(r,t)={P(r,t)-P.sub.0 }/p* (A-7) and take the production (or injection) rate in the form
Q(t)=Q.sub.0 F(t) (A-8) where Q.sub.0 is a positive (or negative) reference flow rate and the dimensionless function F is given. If we now choose
r*=VC/(4 πR.sub.w.sup.2 φc)>0 (A-9)
t*=V.sup.2 C.sup.2 μ/(16 π.sup.2 R.sub.w.sup.4 kφc)>0(A-10)
p*=VCQ.sub.0 μ/(16 π.sup.2 R.sub.w.sup.4 kφc) (A-11) the boundary value problem reduces to
∂.sup.2 p(r,t)/∂r.sup.2 +2/r∂p/∂r=∂p/∂t(A-12)
p(r,0)=0 (A-13)
p(∞,t)=0 (A-14)
∂p(r.sub.w,t)/∂r-∂p/∂t=F(t) (A-15) which is free of formulation parameters, except for the single dimensionless radius r.sub.w >0 given by
r.sub.w =4 πR.sub.w.sup.3 φc/(VC) (A-16) Solution using Laplace transforms. In order to solve this problem, we introduce the Laplace transform
p(r,s)=∫exp (-st)p(r,t)dt (A-17) where the t integration is taken over 0<t<∞, and limits are omitted for brevity, and s>0 is required in order that the integral exist. If we multiply Equation A-12 by exp(-st) throughout and perform the suggested (0,∞) integration, simple integration-by-parts and transform table look-up leads to
d.sup.2 p(r,s)/dr.sup.2 +2/rdp/dr=sp-p(r,0). (A-18) Similarly, Equation A-14 leads to
∫exp (-st)p(∞,t)dt=∫exp (-st)0dt=0 or
p(∞,s)=0 (A-19) while Equation A-15 becomes
dp(r.sub.w,s)/dr-sp+p(r.sub.w,0)=F(s) (A-20) where F(s)=∫ exp(-st) F(t) dt is the Laplace transform of F(t). Now, since Equation A-13 requires that p(r,0)=0, Equations A-18 and A-20 respectively simplify to
d.sup.2 p(r,s)/dr.sup.2 +2/rdp/dr-sp=0 (A-21)
dp(r.sub.w,s)/dr-sp=F(s). (A-22) Equation A-21 is a special case of Bessel's equation having the solution
p(r,s)=r.sup.-1/2 {C.sub.1 I.sub.1/2 (rs.sup.1/2)+C.sub.2 I.sub.-1/2 (rs.sup.1/2)} (A-23) where the Bessel functions I.sub.1/2 and I.sub.-1/2 are conventionally given as
I.sub.1/2 (rs.sup.1/2)=√{2/(πrs.sup.1/2)} sin h (rs.sup.1/2)(A-24)
I.sub.-1/2 (rs.sup.1/2)=√{2/(πrs.sup.1/2)} cos h (rs.sup.1/2).(A-25) However, in applying the farfield regularity condition, we observe that the functions sinh (rs.sup.1/2) and cosh (rs.sup.1/2) both increase indefinitely as is impossible to directly satisfy p(∞,s)=0. In order to use the solution in Equation A-23, we need to recognize that sinh (rs.sup.1/2)=1/2 {e.sup.r√s -e.sup.-r√s } and cosh (rs.sup.1/2)=1/2{e.sup.r√s +e.sup.-r√s }, which allows us to equivalently express Equation A-23 in the form
p(r,s)=r.sup.-1 {C.sub.3 e.sup.r√s +C.sub.4 e.sup.-r√s }(A-26) where the "s"'s 's of Equations A-24 and A-25 have been absorbed into the definition of C.sub.3 and C.sub.4. Now, the requirement from Equation A-19 that p(∞,s)=0 is easily enforced by taking C.sub.3 =0, which leaves
p(r,s)=C.sub.4 r.sup.-1 e.sup.-r√s. (A-27) Substitution of Equation A-27 in Equation A-22 leads to an expression for the integration constant C.sub.4, namely
C.sub.4 (s)=-{F(s)r.sub.w.sup.2 exp (r.sub.w s.sup.1/2)}/{r.sub.w s+r.sub.w s.sup.1/2 +1}. (A-28) Hence, Equation A-27 takes the final form
p(r,s)=-{F(s)r.sub.w.sup.2 exp (r.sub.w s.sup.1/2)}{r.sup.-1 exp (-rs.sup.1/2)/{r.sub.w s+r.sub.w s.sup.1/2 +1{ (A-29) which applies to all values of r. In this appendix, though, we will only be interested in values of r at the sandface, that is, at r=r.sub.w. Evaluating Equation A-29 there, we have the final pressure transform at the well
p(r.sub.w,s)=-F(s)/(s+s.sup.1/2 +r.sub.w.sup.-1). (A-30) Build-up or drawdown example. We illustrate the solution technique by considering the simple case when
Q(t)=Q.sub.0 F(t)=Q.sub.0 (A- 31) that is, F(t)=1. where Q.sub.0 >0 for production (pressure drawdown) and Q.sub.0 <0 for production (pressure buildup). Thus, it follows that
F(s)=1/s (A-32)
p(r.sub.w,s)=-1/{s(s+s.sup.1/2 +r.sub.w.sup.-1)}. (A-33) Equation A-33 can be more conveniently written using partial fraction expansions as ##EQU14## where the complex constants β.sub.1, and β.sub.2 satisfy
β.sub.1 =+1/2-1/2√(1-4r.sub.w.sup.-1) (A-35)
β.sub.2 =+1/2+1/2√(1-4r.sub.w.sup.-1). (A-36) If we now apply the transform-inverse relationship
1/(β+s.sup.1/2))β.sup.-1 erfc (0)-β.sup.-1 exp (β.sup.2 t) erfc (β√t) (A-37) and use the fact that
erfc (0)=1 (A-38) we obtain the exact dimensionless transient pressure function as
p(r.sub.w,t)={1/(β.sub.1 -β.sub.2)}{β.sub.1.sup.-1 -β.sub.1.sup.-1 exp (β.sub.1.sup.2 t) erfc (β.sub.1 √t)-β.sub.2.sup.-1 +β.sub.2.sup.-1 exp (β.sub.2.sup.2 t)erfc(β.sub.2 √t)} (A-39) which is exact for all time t and all r.sub.w. Validation. To show that this reduces to known conventional results at small and large times, we can introduce small time Taylor series and large time asymptotic expansions for the exponential and complementary error functions in Equation A-39. This straightforward procedure yields, on returning to dimensional variables,
P(R.sub.w,t)=P.sub.0 -Q.sub.0 t/(VC) (A-40) at small times, and reproduces a known linear dependence on time that depends on flow-line storage properties only. For production, Q.sub.0 >0 leads to pressure drawdown, while the pressure buildup is consistently obtained for the injection limit. For large times, Equation A-39 reduces to
P(R.sub.w,t)=P.sub.0 -Q.sub.0 μ/(4 πR.sub.w k)+{Q.sub.0 μ/(4 πk)}√{φμc/(πkt)} (A-41) which is independent of flow-line storage, depending only upon transport properties such as viscosity and permeability. Equation A-41 also reproduces the known algebraic "square-root" decline in pressure. If equation A-33 is simplified by eliminating the lower order term s.sup.1/2, then the Laplace space solution becomes
p(r.sub.w,s)=-1/{s(s+r.sub.w.sup.-1)} (A-42) Equation (A-42) can be more conveniently written as
p(r.sub.w,s)=-r.sub.w {1/s-1/(s+r.sub.w.sup.-1)} (A-43) Now applying inverse Laplace transforms we obtain
p(r.sub.w,t)=r.sub.w {1-e.sup.-(t/rw) } (A-44) Exponential equations similar to A-44 have been used to model wellbore storage. Their derivations are based on empirical observations of typical well bore behavior and are not scientifically rigorous (i.e., van Everdingen, A. F.: "The Skin Effect and Its Influence on the Production Capacity of a Well," Trans., AIME (1953) 198, 171 and Hurst, W.: "Establishment of the Skin Effect and Its Impediment to Fluid Flow into a Wellbore," Pet. Eng. (Oct. 1953) B6.). To the best of our knowledge, the rigorous derivation of equation A-44 result starting from first principles has not been presented in the literature. The derivations given here show that equation A-44 itself represents an approximation to the more complete and exact solution derived earlier. Now we observe that equation A-41 expressed in dimensionless units becomes ##EQU15## Combining equations A-43 and A-45 yields the approximate formula that closely matches the exact solution given in equation A-39. ##EQU16## The formulas in equations A-44 and A-46 are convenient for fast, approximate calculations, but when high accuracy is required, the exact solution in equation A-39 can be used. The above formulation, identical to Brigham's classic wireline formation tester model ("The Analysis of Spherical Flow with Wellbore Storage," SPE Paper 9294, 1980), among others, assumes constant total flow rate injection (or production), where this net flow consists of sandface flow rate and storage expansion effects. In practice, constant rates may not be realizable in downhole testers, thus time-dependent rates F(t) should be considered instead. The following explains how the exact solution has been extended to handle arbitrarily prescribed flow rates. Using the same dimensionless notation as before, the pressure solution governing the more general problem is ##EQU17## This reduces to earlier results when F(t)=1, and as before, implicitly contains all "type curve" results that are conventionally given numerically in plots and tables. This section presents a solution for the same spherical flow boundary value problem presented previously with the advantage of solving the problem for all values of r (new nomenclature is introduced where appropriate). This solution enables remote monitoring probes in the reservoir to be used for pressure buildup analysis in addition to the sink probe. Another advantage of this second solution is the fact that conventional dimensionless parameters are used that are familiar to petroleum engineers. This solution follows the same problem formulation and conventions of Brigham's classic wireline formation tester model ("The Analysis of Spherical Flow with Wellbore Storage," SPE Paper 9294, 1980). Restating the basic spherical flow partial differential equation: ##EQU18## The dimensionless parameters are defined in a similar manner as in Brigham's paper: dimensionless radius: ##EQU19## dimensionless time: ##EQU20## dimensionless pressure: ##EQU21## dimensionless storage: ##EQU22## where r.sub.sw is defined as the pseudo spherical wellbore radius and Q is a constant volume flow rate. Introducing the dimensionless identifies into the spherical flow equation transforms equation (A-48) into the dimensionless form: ##EQU23## Brigham introduced the additional dimensionless variable b.sub.d, a product of p.sub.d and r.sub.d to facilitate the solution: ##EQU24## Substituting equation (A-54) into equation (A-53) yields: ##EQU25## The initial and outer boundary conditions may be stated as follows: ##EQU26## and the inner boundary condition is: ##EQU27## Talking the Laplace transform of the partial differential equation (A-55), it is converted to an ordinary differential equation: ##EQU28## Then the general solution of this equation for an infinite system can be stated as: ##EQU29## where C.sub.1 is an arbitrary constant. Brigham shows that the particular solution in Laplace space can be derived by: ##EQU30## From this point Brigham and others used numerical techniques to invert equation (A-61). An exact solution is instead developed by examining this equation and applying an inverse Laplace transform. By rearranging terms in equation (A-61), the Laplace space solution can be expressed as follows: ##EQU31## Equation (A-62) can be rewritten as: ##EQU32## Using the inverse Laplace transform from "The Handbook of Mathematical Functions" by M. Abramowitz and I. A. Stegun (10th printing, December 1972, Equation 29.3.89, page 1027): ##EQU33## Equation (A-65) can now be solved and the exact spherical flow solution stated as: ##EQU34## This equation is a general solution for the entire formation and can be used for all values of wellbore radius greater than the pseudo spherical wellbore radius or sink radius (i.e., r>r.sub.sw). If this equation is evaluated at the pseudo spherical wellbore radius (i.e., r=r.sub.sw), this solution becomes identical to the solution presented in equation A-39 with the advantage of using standard dimensionless parameters such as C.sub.D and r.sub.D (equation A-39 is advantageous in that no storage or radius parameters explicitly appear). Other improvements can be made to this solution to include anisotropy and skin corrections. These improvements follow the same procedures used by Brigham and others. The main advantage of using equation (A-62) as well as other equations that can be easily derived from it, is that it is an exact solution which makes parameter matching in the data collection and computing systems much faster and more accurate than methods currently used in wireline formation tester logging systems. Another form of the exact solution shown in equations A-49, A-50, A-51, and A-67 can be developed for the complete drawdown and buildup process. Equation A-49 can be expressed as: ##EQU35## This more general form, valid for multiple spaced points, has the advantage of using measured date from several pressure probes. This can be used to approximate the ratio of horizontal to vertical permeability or formation anisotropy. ##SPC1## The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: FIG. 1 is a schematic representation of the primary components of a wireline formation tester positioned in a borehole to collect data relating to parameters of the surrounding earth formation; FIG. 2 is a graphical representation of expected pressure measurements vs. time during the drawdown and buildup cycles of a formation test, illustrating the variation of pressure measurements as a function of the flow line storage effects in three exemplary cases; FIG. 3 is a graphical representation of pressure measurements vs. time of a typical pretest sequence performed by the formation tester; FIG. 4 is a graphical representation of typical pressure measurements vs. time of a typical pretest sequence taken in a permeable zone of a formation by the formation tester having no flow line storage effects; FIG. 5 is a flowchart representing a preferred, electronically-implemented method of determining formation properties using the pressure measurements obtained by a formation tester; FIG. 6 is an exemplary plot of measured pressure data vs. time taken during a first exemplary pretest sequence; FIG. 7 is a magnified portion of the plot illustrated in FIG. 6; FIG. 8 is a plot of measured pressure data, taken from the plot of FIG. 6, vs. time.sup.-1.5 showing a straight line that is curve-fitted to the data; FIG. 9 is a magnified portion of the plot of FIG. 6 showing a type-curve that is curve-fitted to the data in accordance with the present invention; FIG. 10 is an exemplary plot of measured pressure data vs. time taken during a second exemplary pretest sequence; FIG. 11 is a magnified portion of the plot illustrated in FIG. 10; FIG. 12 is a plot of measured pressure data, taken from the plot of FIG. 10, vs. time.sup.-1.5 showing a straight line that is curve-fitted to the data; FIG. 13 is a magnified portion of the plot of FIG. 10 showing a type-curve that is curve-fitted to the data in accordance with the present invention; FIG. 14 is an exemplary plot of measured pressure data vs. time taken during a third exemplary pretest sequence; FIG. 15 is a magnified portion of the plot illustrated in FIG. 14; FIG. 16 is a plot of measured pressure data, taken from the plot of FIG. 14, vs. time.sup.1.5 showing a straight line that is curve-fitted to the data; FIG. 17 is a magnified portion of the plot of FIG. 15 showing a simulated curve that is curve-fitted to the data in accordance with the present invention; FIG. 18 is an exemplary plot of measured pressure data vs. time taken during a fourth exemplary pretest sequence; FIG. 19 is a magnified portion of the plot illustrated in FIG. 18; and FIG. 20 is a magnified portion of the plot of FIG. 18 showing a type-curve that is curve-fitted to the data in accordance with the present invention. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. Citas de patentes
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